This invention relates to a coating composition for metal conductors and to the formation of an electrically insulating coating with improved properties that allow for the removal of specific parts of the coating layer.
In the production of machines, for example, motors, generators, and transformers, wires coated with an electrical insulation are used. These coated wires have electrical contacts attached thereto. Before the electrical contact is attached to the wire, the insulation is cleaned from the wire at the point of contact to enable a flow of electrical current.
The insulating coating material for metallic conductors can be enamels with having a high dielectric strength. In a typical production process, these conductors are entirely coated and subsequently selective portions of the insulating coating are removed to ensure an electrical contact of the respective parts.
The removing of the insulating layer from a coated wire can be done by various methods, for example, mechanical or thermal methods. Both methods require a large amount of time and labor. The mechanical method especially presents a problem, the conductor cross-section may be reduced or deformed .on removal of the coating since there is no guarantee that the coating layer can be precisely removed without damaging the conductor. Any damage to the conductor leads to an undesired increase in electrical resistance when the device is operated.
Apart from these two methods, laser irradiation is a known technique for de-coating or removal of an insulating coating from a conductor. The advantage of using laser irradiation is that it is highly precise and can be positioned on the surface enabling the conductor to de-coat specific portions of the coating. The coating or insulating layer should be able to absorb sufficient laser irradiation to completely evaporate the layer. This method generates a high intensity of energy in an area causing a rapid evaporation of the insulating layer from a particular area of the surface.
A problem with the laser method arises if the respective insulating layer of the conductor has too low an absorption of the laser irradiation whereby only the metal conductor is heated, and only a partial or possibly no de-coating occurs.
Known inorganic fillers, particularly pigments, are able absorb the laser energy, convert it into heat and enable an evaporation of the insulating layer by means of laser irradiation. Examples for such fillers and pigments are the silicates known as glimmer pigments, or the pigments used for the laser marking of plastics. These fillers and pigments are not suitable for the use in an insulating coating since they form a sediment in the coating and form a non-homogeneous coating that has a lower flexibility in comparison to standard coatings and is susceptible to cracking due to changes in elongation of the wire.
U.S. Pat. No. 5,337,941 describes a practical de-coating of a wire insulation by use of a laser. This method is not completely satisfactory since a residue of the insulation layer remains on the conductor. This residue can only be removed completely by the use of a soldering material applied at a high temperature.